<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <script>
        const arr = [45, 21, 356, 66 , 6, NaN, 723, 54];

        console.log(arr.indexOf(66) >= 0)
        console.log(arr.includes(NaN));
    </script>
</body>

</html>